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You must be root to run those commands
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MatApr 27 '12 at 13:44

Manoj Kumar: Did the pkexec method in my answer (which you've marked as accepted) solve this problem for you? I've become convinced that, as Damien Roche and Oli have commented, this method does not actually work after a sudo chmod -R 777 /usr/bin command has successfully completed. Like sudo, pkexec has to be setuid root to work. (I'm not sure why, as it uses the polkit service, but it does.) However, I think I've heard other people say this has worked for them, which is curious! Had you pressed Ctrl+C before the 777 chmod command finished?
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Eliah KaganAug 23 '14 at 1:10

4 Answers
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Accepted status notwithstanding, I'm now convinced this answer is wrong. (I hope to improve it soon, after consulting with the OP about the accept.) I think I recall others saying this worked, but I believe their problems were at least slightly different. The method described herein remains valuable for some situations where chmod -R 777 /usr/bin is interrupted with Ctrl+C or otherwise does not complete. But once it does, pkexec is un-setuid'd too and it won't work any better than sudo, as Damien Roche and Oli have rightly commented.

On an Ubuntu desktop system, PolicyKit is installed, so pkexec can be used to repair a broken sudo executable or sudoers file. You do not need to boot into recovery mode and you do not need to boot from a live CD. You don't even need to reboot.

pkexec must be setuid root! What a nightmare!
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Damien RocheAug 23 '13 at 14:26

1

As Damien points out, the problem with this approach is pkexec is in exactly the same predicament as sudo. It's normally setuid and lives in /usr/bin. I've tested it and this just doesn't work after chmod -R 777 /usr/bin.
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Oli♦Jul 3 '14 at 16:39

@DamienRoche, Oli: I'm sorry for not fixing this way sooner! You're both right. I'm unsure why pkexec has to be setuid root to work (doesn't it work through the polkit daemon like other non-setuid processes?), but it does need that. I've commented on the question to see if the OP (who accepted this) can shed light on what, if anything, is right or helpful about this answer. And I've added a temporary banner to the top of this post so I don't further mislead. If this answer continues to exist, its improved form will probably incorporate some info currently in that banner.
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Eliah KaganAug 23 '14 at 1:34

How to figure out where your disk is mounted: from a terminal window, run mount (without arguments). This will list up all mounted devices. Check for the type listed -- you can skip everything not using a "real file system" (your disk probably uses either ext3 or ext4 -- you can for sure skip things like proc, sysfs and the like). If something sounds promising (looking like /dev/sda1 on /media/sda1 type ext3), check its contents using ls /media/sda1 to see if it's that.

If it is not mounted, you can check with the /dev entries where the disk could be (using ls /dev/ |grep '/dev/sd to check for available devices; your disk should look like /dev/sdaX, /dev/sdbX or the like -- with X being a number). Compare this with the list of mounted devices. If it's not there, try to mount it and check its contents (as shown above). To mount it, first create a mountpoint, e.g. sudo mkdir /mnt/mydisk, then try to mount the device using mount /dev/sda1 /mnt/mydisk and check its contents using ls /mnt/mydisk.

Once you get the right disk there, you can go to change the permissions back on your usr dir: sudo chmod 0755 /mnt/mydisk/usr.

Now you still might be in trouble if you originally ran the chmod command recursively, using the -R parameter. In that case you can either try to fix each entry manually -- or you can go straight for a fresh install...

I tried to create mount point, but I got an error which read mount point mnt/mydisk does not exists is mydisk to be replaced with some thing and just for the record i wrote mount /dev/sda /mnt/mydisk from root@Ubuntu:/dev#
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vinJul 9 '12 at 15:30

You did a) create that mount point (directory) before issuing the mount command (as described) and b) took care for the leading slash as well? Try mkdir -p /mnt/mydisk && mount /dev/sda1 /mnt/mydisk (I don't think you have no partitions on /dev/sda, so you must have missed the partition number as well. Make sure you specify the correct device (or mount will fail).
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IzzyJul 9 '12 at 16:05

I think Mat's correct, you must be root to add the bit to /usr/bin, but of course sudo is broken. If you have a root password you can use that to logon as root and then fix the permissions with the above command. If you don't, however, (and I don't either) it would probably be best to:

boot from a Linux live CD

become root there

mount the partition with the above system

then straighten out the permissions on that file system using a terminal.

Root is always user number 0 so root on any system can make changes allowed to root on other file systems.